Geologic formations are used for many purposes such as hydrocarbon production, geothermal production, and carbon dioxide sequestration. Boreholes are typically drilled into the formations in order to access the formations and perform measurements associated with a proposed use of the formations. One type of measurement is gravity.
Gravity measurements can provide different types of information. Gravity measurements sense far beyond the wellbore s they minimize the effects of the near wellbore environment such as the presence of metal well casing or of fluid invasion, which can significantly alter or block the response of traditional logging tools. Gravity measurements made at two depths can provide the average formation density between those two depths. The average formation density depends not only on the rock type and its porosity but on the fluid that fills the pores of the rock. Gravity measurements that are repeated over time, either in the wellbore or on the surface (often called a “4-D survey”), can allow one to do reservoir monitoring and to observe when water from a water flood has replaced oil or when a gas cap has enlarged as the oil below it was produced. Gravity measurements can be used in old wells through casing to find gas zones that were considered uneconomic at the time the well was drilled. Gravity measurements can also be used in washed out, rugose, or fractured zones, and where the formation was damaged or oxidized.
Another type of information is related to obtaining the true vertical depth of the gravimeter that is measuring gravity in a borehole if one knows, or can estimate, the average formation density from the surface to the gravimeter. As boreholes become deviated from the vertical, it is more difficult to account for the deviation due to wireline stretch or drill pipe bending in estimating true vertical depth. As gravitational acceleration is a function of depth in a formation, the true vertical depth can be determined at any point in a borehole from gravity measurements. Hence, it would be well appreciated in the petroleum industry to develop improved gravimeters and to increase the accuracy of gravity measurements.